Corrosion-resistant alloy

ABSTRACT

The invention relates to metallurgic engineering, to nickel-based alloys intended for use in aggressive oxidizing environments. The invention obtains an alloy with a high level of plastic properties for operation in the temperature range 550° C. to 625° C. and increased corrosion cracking resistance. The alloy contains carbon, silicon, manganese, chromium, molybdenum, phosphorus, sulphur, iron, nickel and unavoidable impurities, titanium, aluminium, niobium, magnesium with specified components amounts.

The invention relates to metallurgic engineering, to nickel-based alloysintended for use in aggressive oxidizing environments.

A corrosion-resistant alloy Nicrofer 6616 hMo alloy C-4 (No. 2.4610),containing wt. %: 14.5-17.5 Cr, 14.0-17.0 Mo, ≤3.0 Fe, ≤0.009 C, ≤1.0Mn, ≤0.05 Si, ≤2.0 Co, ≤0.7 Ti, ≤0.020 P, ≤0.010 S, nickel and otherunavoidable impurities is known from the prior art (Catalogue“Corrosion-resistant, heat-resistant and high-strength steels andalloys”, M., Prometey-Splav, 2008, pp. 304-306).

The alloy is used for the manufacture of equipment operated in a widerange of chemical environments, at room and elevated temperatures. Inparticular, for adsorbers in flue gas desulphuring; etching baths andacid recovery plants; acetic acid and agrochemicals plants.

The nearest analogue of the given invention is an alloy XH65MBY(

Π760) containing, wt. %: ≤0.02 C, ≤0.1 Si, ≤1.0 Mn, 14.5-16.5 Cr,15.0-17.0 Mo, 3.0-4.5 W, ≤0.5 Fe , ≤0.012 S, ≤0.015 P, nickel and otherunavoidable impurities (GOST 5632-2014 - prototype).

The alloy is used for the manufacture of welded structures (columns,heat exchangers, reactors) operating under elevated temperatures inaggressive redox environments, in the chemical, petrochemical industry(production of acetic acid, epoxy resins, vinyl acetate, melamine,complex organic compounds) and other industries in the temperature range−70 to 500° C.

The XH65MB alloy and its welded joints can be used in KCl—AlCl3-ZrCl4media only up to 500° C., because at a temperature above this value, thealloy, in addition to intergranular corrosion and corrosion cracking,sharply decreases the percentage elongation from 48% to 7.3-13% at 550°C. and up to 2.5% at 625° C. and the embrittlement of the metal appearswhen deformation is applied.

The objective of the invention is to create an alloy having a high levelof corrosion properties at temperatures up to T=650° C. in the workingmedia of chloride plants (KCl—AlCl3-ZrCl4).

The technical result of the invention is to obtain an alloy with a highlevel of plastic properties for the operation in the temperature range550° C. to 625° C. and increased corrosion cracking resistance inchlorides KCl, AlCl3+(ZrCl4 HfCl4) molten metal, at temperatures up to650° C.

The specified technical result is achieved in that the alloy containingcarbon, silicon, manganese, chromium, molybdenum, phosphorus, sulphur,iron, nickel and unavoidable impurities, according to the inventionadditionally contains titanium, aluminium, niobium, magnesium with thefollowing components ratio, wt. % :

Carbon ≤0.006 Silicon ≤0.1  Manganese ≤1.0  Chromium 22.8-24.0 Iron≤0.75  Molybdenum 12.0-14.0 Niobium 0.01-0.03 Titanium 0.01-0.06Aluminium 0.1-0.2 Magnesium 0.005-0.01  Phosphorus ≤0.015 Sulphur ≤0.012Nickel and unavoidable impurities balance

To obtain a stable structure and plastic properties, it is preferablethat the content of chromium, molybdenum and iron is related by theratio:

$\begin{matrix}{{\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},4} & (1)\end{matrix}$

(the ratio of the total weight percentage of chromium and molybdenum tothe percentage of iron is not less than 46.4)

To obtain a stable structure and high corrosion properties, it ispreferable that the content of niobium and carbon is related by theratio:

$\begin{matrix}{{\frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack} \geq 1},66} & (2)\end{matrix}$

(the ratio of the weight percentage of niobium to the weight percentageof carbon is not less than 1.66).

It is preferably that the content of chromium, molybdenum, iron, niobiumand carbon is related by the ratios:

$\begin{matrix}{{\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},4} & (1) \\{{{{At}\mspace{14mu} \frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack}} \geq 1},66.} & (2)\end{matrix}$

Comparative analysis with the prototype allows making a conclusion thatthe claimed alloy differs from the known one with a lower carbon content(≤0.006% instead of ≤0.02), molybdenum (12.0-14.0% instead of15.0-17.0%), increased chromium content (23.0-24.0% instead of14.5-16.5%), iron (≤0.75% instead of ≤0.5%) does not contain tungsten,as well as with the additional introduction of elements such as niobiumin an amount of 0.01-0.03%, titanium in an amount of 0.01-0.06%,aluminium in an amount of 0.1-0.2% and magnesium in an amount of0.005-0.01%.

Moreover, in particular cases of the invention, the claimed ratios ofelements are observed:

${\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},{4;}$or ${\frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack} \geq 1},66$${{{Or}\mspace{14mu} \frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack}} \geq 46},{{4\mspace{14mu} {at}\mspace{14mu} \frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack}} \geq 1},66.$

The limits of the content of alloying elements in the invention alloyare specified as a result of a study of alloys properties with differentcomposition options.

Exceeding the carbon content of more than 0.006% leads to a decrease incorrosion resistance in solutions of zirconium and hafnium salts due toan increase in the carbide formation process at high temperatures (theappearance of undesirable carbide phases).

The chromium content was found to be 22.8-24.0% to ensure the requiredheat resistance in hafnium and zirconium oxides. When chromium isintroduced into the alloy in the amount of less than 22.8%, the requiredheat resistance is not ensured, and exceeding the content above 24.0%impairs the heat resistance of the alloy.

The introduction of molybdenum into nickel alloys increases therecrystallization temperature of solid solutions, inhibits theirsoftening, increases heat resistance, and leads to an ductility increaseduring short and long tests.

The range of molybdenum content of 12.0-14.0% is selected to provide therequired mechanical properties for both short-term and long-term loadsand high temperatures. With the introduction of less than 12.0% ofmolybdenum, the mechanical properties are not met. When the content isabove 14.0%, there is a decrease in ductility and, accordingly, adecrease in the processability of the alloy during metallurgicalprocessing.

Niobium in an amount of 0.01-0.03%, binds residual carbon and nitrogento carbides, nitrides and carbonitrides, prevents the formation ofchromium carbides and carbonitrides along the grain boundaries. Theaddition of niobium in an amount 6 to 10 times higher than the carboncontent in the alloy eliminates intergranular corrosion of the alloysand protects the welds from destruction. When the niobium content isless than 0.01%, its interaction with residual carbon is ineffective,and the niobium content above 0.03% is not reasonable for carbideformation.

Exceeding the silicon content of more than 0.1% negatively affects theprocessability of the alloy, as well as leads to embrittlement of thealloy due to an increase of silicon silicates content in it.

Increase of manganese content over 1.0% leads to the appearance of afusible eutectic, which leads to the destruction of the ingot duringpressure processing and reduces the heat resistance of the alloy, aswell as leads to a decrease of local corrosion resistance.

Nickel is stable in HCl even at boiling point. However, in the presenceof chlorides, ions of Fe(III) and other oxidizing agents corrosion ofnickel and nickelchrome molybdenum alloys is enhanced, the limitation ofthe iron content of not more than 0.75% is due to this. The introductionof titanium in an amount of 0.01-0.06% increases the corrosionresistance in melts of zirconium and hafnium salts, binds residualcarbon to carbides and leads to the formation of a sufficient amount ofNi3Ti type intermetallic compound, which, at an operating temperature of500-700C, positively affects the heat resistance of the alloy. When thetitanium content is less than 0.01%, the requirements for corrosionresistance are not met, and the excess of the titanium content above0.06% leads to a decrease in the processability of the alloy and theformation of undesirable phases due to the reactivity of titanium.

Aluminium and magnesium in the amount of 0.1-0.2% and 0.005-0.01% areintroduced into the alloy to remove residual oxygen, as well as, withregard to aluminium, to form an intermetallic compound of the Ni3Altype, which positively affects the heat resistance of the alloy. Whenthese elements are introduced in amounts less than specified, thenecessary removal of residual oxygen is not achieved. If the content ofthese elements is exceeded, gross non-metallic inclusions are formed.

When the sulphur content exceeds 0.012% and phosphorus exceeds 0.015%,coarse non-metallic inclusions are formed that adversely affect theductility of the alloy.

Under the condition

${\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},4,$

when the ratio decreases below 46.4, the alloy structure becomes lessstable (sigma phase is released), which has a negative effect on plasticcharacteristics and corrosion resistance.

In the condition

${\frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack} \geq 1},66,$

with a ratio of less than 1.66, a decrease in the corrosion resistanceof the alloy occurs.

The proposed ratio of the elements in the alloy were foundexperimentally and are optimal, since they allow obtaining the claimedcomprehensive technical result. When breaking the ratios of theelements, the properties of the alloy deteriorate, their instability isobserved, and the complex effect is not achieved.

Examples of the invention implementation.

Alloy ingots were smelted in vacuum induction furnaces. The change inthe plastic properties of the studied alloys under the influence oftemperatures of 550° C. and 625° C. after long exposure in the furnacefor more than 1000 hours was controlled by bending samples to an angleof 90 degrees or more according to GOST 14019-2003. Industrial corrosioncracking resistance tests of alloys were carried out in molten chloridesKCl, AlCl3 +(ZrCl4 HfCl4)

Table 1 shows the chemical composition of alloy ingots with variouscompositional options, as well as the prototype alloy. Table 2 shows theresults of determining the plastic properties of the alloys indicated intable 1 by bending at an angle of 90 degrees according to GOST14019-2003. Table 3 presents the results of industrial corrosioncracking resistance tests of the alloys indicated in Table 1 in moltenchlorides KCl, AlCl3 +(ZrCl4 HfCl4), 100 hours, at T=650° C.

As can be seen from tables 1, 2, the plastic properties of alloy at 550and 625 C with the claimed composition (alloys 1, 2) are higher than theproperties of the prototype alloy, alloy 3, not satisfying the claimedcomposition, has lower plastic characteristics than alloys 1, 2 , whichleads to the formation of cracks as a result of bending tests accordingto GOST 14019-2003.

As it can be seen from table 3, the corrosion rate of alloys (alloys 1,2) that satisfy the claimed composition is lower than the corrosion rateof the prototype alloy, visual inspection did not reveal the cracks,unlike the prototype alloy. The corrosion rate of alloy 3, which doesnot satisfy the claimed composition, exceeds the corrosion rate ofalloys 1, 2 (however, lower than the corrosion rate of the prototypealloy), visual inspection revealed a crack in the sample.

TABLE 1 Chemical composition of the investigated alloys Ni andunavoidable Alloy C Mn Si Mo Cr Nb S P Fe Ti Al W impurities Ratio (1)Ratio (2) Alloy 1 0.0011 0.55 00.

3.0 23.3

0.03

.0028

0.01

.54 ≤0.01 0.

— balance 67.2 27.27 Alloy 2 0.0058 0.31 0.10 3.1 22.9

0.02 0.005 0.00

.75 0.05 0.

— balance 48.0 3.45 Alloy 3 0.009 0.65 0.10 2.5 23.6

0.01 0.008 0.00

.84 0.04 0.

— balance 42.98 1.11 Alloy acc. 0.017 0.63 0.08 6.2 5.6 — 0.006 0.00

.45 — — 3.7 balance — — to prototype

indicates data missing or illegible when filed

TABLE 2 Results of determining plastic properties by bending at an angleof 90 degrees according to GOST 14019-2003 Exposure temperature, ° C.550° C. 625° C. Exposure Samples Exposure Sample Alloy time, h resultbending Time, h result bending Alloy acc. to 720 Sample broken 720Sample broken prototype 1000 No cracks 1000 Crack 2065 Crack 2065 CrackAlloy 1 720 No cracks 720 No cracks 1000 No cracks 1000 No cracks 2065No cracks 2065 No cracks Alloy 2 720 No cracks 720 No cracks 1000 Nocracks 1000 No cracks 2065 No cracks 2065 No cracks Alloy 3 720 Nocracks 720 No cracks 1000 No cracks 1000 Crack 2065 Crack 2065 Crack

TABLE 3 Results of industrial corrosion cracking resistance tests ofalloys in chloride melts KCl, AlCl3 + (ZrCl4 HfCl4), 100 h, at T = 650C. Visual inspection Corrosion rate, Alloy Cracks after testing mm/yearAlloy acc. to Crack in sample 0.50 prototype Pit corrosion in a sampleup to 0.1-0.2 mm deep Alloy 1 No cracks 0.16 Pit corrosion in metalsample up to 0.1-0.2 mm deep Alloy 2 No cracks 0.21 Pit corrosion in asample up to 0.1-0.2 mm deep Alloy 3 Crack in sample 0.45 Pit corrosionin a sample up to 0.1-0.2 mm deep

1. A corrosion-resistant nickel-based alloy containing carbon, silicon,manganese, chromium, molybdenum, phosphorus, sulphur, iron, nickel andunavoidable impurities, wherein it additionally contains titanium,aluminium, niobium, magnesium with the following components ratio, wt.%: Carbon ≤0.006 Silicon ≤0.1  Manganese ≤1.0  Chromium 22.8-24.0 Iron≤0.75  Molybdenum 12.0-14.0 Niobium 0.01-0.03 Titanium 0.01-0.06Aluminium 0.1-0.2 Magnesium 0.005-0.01  Phosphorus ≤0.015 Sulphur ≤0.012Nickel and unavoidable impurities balance


2. The alloy according to claim 1, wherein the content of chromium,molybdenum and iron is related by the ratio:${\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},4.$3. The alloy according to claim 1, wherein the content of niobium andcarbon is related by the ratio:${\frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack} \geq 1},66.$
 4. The alloyaccording to claim 1, wherein the content of chromium, molybdenum andiron is related by the ratio:${\frac{\lbrack{Cr}\rbrack + \left\lbrack {\overset{\prime}{I}\hat{i}} \right\rbrack}{\lbrack{Fe}\rbrack} \geq 46},4$and the content of niobium and carbon is related by the ratio:${\frac{\lbrack{Nb}\rbrack}{\lbrack C\rbrack} \geq 1},66.$